In recent years, in an effort aiming at an optical recording medium having a still larger recording capacity and density, a volume hologram optical recording medium has been developed, which records information as a hologram by changing the refractive index of a recording layer in accordance with an optical intensity distribution generated by the interference of light.
The general principle about forming a hologram is described in several technical documents or books (see Non-Patent Document 1). According to these, an object to be recorded is irradiated with one of two coherent laser beams of light, while a photosensitive hologram recording material is placed at the position where the laser beam is receivable.
A hologram recording material is irradiated with a direct coherent light which does not illuminate the object, in addition to a light from an object. The light from the object is called an object light, and the light that directly illuminates the recording material is called a reference light. Interference patterns between the reference light and the object light is recorded as image information. Then, when the recording material treated as above is irradiated with the same light (reproduction light) as the reference light, the light is diffracted by holograms in a manner that the wave surface of the reflected light, which has originally arrived at the recording material from the object in the recording step, is reproduced. As a result, an object image similar to the real image of the object is observable three-dimensionally.
A hologram formed by irradiating the hologram recording material with the reference light and object light from the same direction is called a transmission hologram, while a hologram formed by irradiating the recording material with the reference light and the object light from an opposite direction is called a reflection hologram. A hologram having a sufficient film thickness with respect to the spacing of the interference stripes (usually 5 times or more of the spacing of the interference stripes or a thickness of about 1 μm or more) is called a volume hologram. Recording with a higher density is attainable with a thicker film, because the recording is performed toward the film thickness direction.
An example of known recording materials for a volume phase hologram is a write-once type hologram that does not need wet processes or bleaching processes. The composition thereof is usually a resin matrix with a photoactive compound incorporated. Examples thereof include a photopolymer system that is a combination of a resin matrix and radical or cation polymerizable monomers (see Patent Document 1).
Upon recording information, when the recording layer is irradiated with the object light and reference light, interference patterns consisting of light portions and dark portions are formed in the recording layer. For example, when the photoactive compound is a radical polymerizable compound, a photo-radical polymerization initiator absorbs light and changes into a radical active species in the light portions. This radical active species causes addition reaction with neighboring radical polymerizable compounds, and the resultant addition reaction product changes into a radical active species. Further, this addition reaction product that changes into a radical active species causes addition reaction with neighboring radical polymerizable compounds. The polymer of the light portions is formed in the recording layer by repeating this series of photo polymerization reaction.
On the other hand, with the advancement of the polymerization reaction in the light portions, a concentration gradient of radical polymerizable compounds develops. The radical polymerizable compounds in the dark portions of the recording layer move by diffusion to the light portions. In contrast, other components in the light portions move by diffusion to the dark portions. Whereby, the light portions and dark portions in the interference patterns become to be composed of different compounds from each other, so that the light portions and dark portions exhibit different refractive indexes from each other.
As a result, the volume hologram optical recording medium keeps this refractive index difference as information. As the refractive index difference is larger, a larger diffraction efficiency is obtained. Therefore, in order to develop a difference in the refractive index, attempts have been made to use a compound having an aromatic ring, a heterocyclic ring, chlorine, bromine or the like for either one of the resin matrix or monomers.
The photopolymer system is generally composed of a basic composition including a resin matrix, a photoactive compound, and a photo polymerization initiator, and is a practical and promising system having a capability of satisfying both of a high diffraction efficiency and a dry process treatment. However, a system having the optimal sensitivity upon recording, a sufficient diffraction efficiency, a high S/N ratio and a high multiplicity has been requested, and a system exhibiting the excellent performances in the stability of signal recording and reliability has been also desired. To meet the requests, various attempts have been made on the formulation of a composition for recording or the method of producing the medium.
For example, in Patent Document 2, a chain transfer agent or a polymerization inhibitor is used. The examples of Patent Document 2 describe the addition of a compound that is generally called as a chain transfer agent or a polymerization retardant, represented by phenols, allyl compounds, and amine compounds.
However, although influences were attained on the retardation of photo reaction and the extension of induction period before polymerization starts by the addition of these compounds, effects on the suppression of temporal change of recorded signals and on the improvement of S/N ratio were not sufficient.
Patent Document 3 describes that 2,6-di-tert-butyl-4-methylphenol (BHT) is added in a volume hologram recording medium.
BHT is considered to be added with an expectation of allowing it to serve as an agent for polymerization retardation and chain transfer. However, any effect on the variation of signal intensity and the stability and reliability of the medium is not described.
Patent Document 4 describes a system of adding a tertiary amine in a volume hologram recording medium. The reason why the tertiary amine is added is not clearly described, but Patent Document 4 describes that the sensitivity and recording quality can be improved.
However, there is no specific description about signal quality or the like. In addition, it is concerned that the durability of the medium is decreased because amines are added to the medium composition.
Patent Document 5 describes a volume hologram recording medium having an oxygen permeation layer.
Patent Document 5 describes that the recording quality is improved and a medium having an excellent multiple recording performance is obtained because oxygen that penetrates into the medium terminates radical polymerization reaction.
However, oxygen does not easily penetrate into the medium uniformly so that the effect is often not exerted. In addition, it is concerned that the oxygen and moisture that penetrate into the medium shorten the life of the medium, so that the expected effect is considered not to be exerted sufficiently.
Non-patent document 1: “HOLOGRAPHIC DISPLAY”, section 2, written by Junpei Tujiuchi, published by Sangyo Tosho
Patent document 1: Japanese Patent No. 3737306
Patent document 2: International Publication No. WO 2006/093846, brochure
Patent document 3: Published Japanese Translation of a PCT Patent Application No. 2004-537620
Patent document 4: International Publication No. WO 2005/078532, brochure
Patent document 5: Japanese Patent Application Laid Open Publication No. 2006-267596